1. Field of the Invention
The present invention generally relates to the fabrication of electronic microcircuits, and more specifically to a method and apparatus for validating proper performance of a microcircuit fabrication procedure.
2. Description of the Related Art
A method for protecting hardware integrated circuits or microcircuits against unauthorized copying and/or use is described in U.S. Pat. No. 4,766,516, entitled "Method and Apparatus for Securing Integrated Circuits from Unauthorized Copying and Use", issued Aug. 23, 1988, to F. Ozdemir et al, and assigned to Hughes Aircraft Company, the assignee of the present invention. According to this technique, at least one additional circuit element that does not contribute towards the function of an integrated circuit (IC), but rather inhibits the proper functioning of the IC in case of an attempted copying, analysis, or other unauthorized use, is fabricated along with the overall IC. The identities of the additional circuit elements are disguised by forming them with the visible appearance of apparent elements, but with physical modifications that are not readily visible to a copyist but cause them to function in a different manner. Such modifications include very narrow open circuit cuts in metallized connection lines, preferably made with a focused ion beam (FIB) or laser beam; disordering the lattice structure or changing the doping level of a semiconductor region, preferably with a FIB; and injecting electrical charge into a semiconductor region, preferably with an electron beam.
The FIB and other types of modifications are relatively undetectable, which is desirable for the intended purpose of deterring unauthorized copying and/or use. However, the undetectability of the modifications also makes it difficult to determine if the modification procedure was performed properly. A malfunction in the apparatus or controlling software, as well as unauthorized alteration or use thereof, could render the modifications or the circuit itself inoperative or different from what was intended.
In addition to security applications, FIBs, for example, are used in a wide variety of microcircuit fabrication processes, an example being presented in an article entitled "Lithographic approach for 100 nm fabrication by focussed ion beam", by S. Matsui et al, Journal of Vacuum Science Technology, B4(4), pp. 845-849 (Jul/Aug 1986). Erroneous performance of an FIB processing procedure will produce defective microcircuits, with the source of the error being difficult to determine due to the relative undetectability of the FIB modifications.